Field of the Invention
The present invention relates generally to magnetic memory systems having magnetic tunnel junctions (MTJs) accessed by a transistor for writing to and reading thereof and particularly to the programming or writing of the MTJ using the transistor.
Description of the Prior Art
Magnetic random access memory (MRAM) is rapidly gaining notoriety as its use in replacing conventional memory is showing promise. Magnetic tunnel junctions (MTJs), which are essentially the devices storing information, include various layers that determine the magnetic behavior of the device. An exemplary MTJ uses spin torque transfer to effectuate a change in the direction of magnetization of one or more free layers in the MTJ. That is, writing bits of information is achieved by using a spin polarized current flowing through the MTJ, instead of using a magnetic field, to change states or program/write/erase/read bits. Currently, the problem with MRAMs are their size. Obviously, reducing the size of a MRAM cell is highly desirable.
Moreover, increasing memory capacity by stacking more than one MTJ on top of another provides great value in terms of costs and real estate on a semiconductor or chip. To this end, writing to or programming of stacked or multi-state MTJs is needed. The current state of technology does not allow for writing or programming a stack of MTJs.
One of the problems facing the programming of MTJs is the switching of the magnetic orientation of the MTJ from an anti-parallel to a parallel state, or generally what is referred to as the programming of a binary “0” value although, this could also be viewed as the programming of a logical “1” value in some instances. One particular problem associated with an anti-parallel to parallel state programming in some programming techniques, particularly those that have tried to address the problem of increasing the drive current through the access transistor that is coupled to the MTJ is that Vgs, the drain to source voltage of the access transistor, exceeds Vcc causing a voltage (or voltage bias) violation of the access transistor. That is, transistors, particularly those made with thin oxide, operate reliably when the difference between their gate and source is less than a Vcc voltage and the voltage difference between their drain and source is less than the Vcc voltage and the voltage difference between their gate and drain is less than Vcc voltage. Vcc is a voltage generated by a power supply that is the maximum voltage applied to any of the gates of a transistor, such as the gate, source and drain. During the programming of an MTJ, currently, the anti-parallel to parallel transition, at times, undesirably results in the voltage difference between the gate and the source of the access transistor exceeding Vcc, which results in the eventual destruction of the transistor.
Thus, the need arises for the MTJs of a magnetic memory array to be programmed or written to reliably.